Polychromators are fitted into emission spectrometry instruments for analyzing the polychromatic light emitted by exciting the sample to be analyzed. These polychromators make use, most frequently, of the Paschen-Runge mounting arrangement and comprise a concave, engraved or holographic diffraction grating which receives the polychromatic light through an inlet slit in order to split it up, by diffraction, into a series of convergent bundles of monochromatic light. Some of these bundles are isolated or selected by outlet slits, placed at the level of the images of the inlet slit. In the Paschen-Runge mounting arrangement, the inlet slit, the grating and the outlet slits are situated on a notional circle, the so-called Rowland circle, which is tangential to the grating and has a diameter equal to the radius of curvature of the grating. The light fluxes are measured by detectors consisting of photomultipliers.
In emission spectrometry, the analysis lines are located mainly in the ultraviolet region. There is also a demand for analyzing the alkali metal elements, such as sodium, lithium, potassium, rubidium and cesium, the lines of which are located in the visible and near infra-red regions (from 5889 .ANG. to 8521 .ANG.).
For obtaining a spectrum extending from the ultraviolet to the near infra-red, a Paschen-Runge mounting arrangement having a great focal length (of the order of 1 meter) and a grating having 1000 to 1500 of lines per millimeter are generally used. As a result, the linear dispersion coefficient (number of nanometers per unit length of spectrum), which is inversely proportional to the number of lines per millimeter and to the focal length, is relatively high. The use of a single grating for such a wide wavelength range is therefore not advantageous.